Dortmund sporting director Sebastian Kehl said: "He had a successful operation last week.

"The operation went very, very well and he's on the right track."

Kehl added that treatment plans are still being discussed and a return date had yet to be fixed.

"Sebastien will be out for a few months but once we can be more precise about that we will communicate it."

Haller joined the Bundesliga side from Ajax earlier this month for 31m euros after a short spell at West Ham.